docs: def from_file(cls, f):
          """Load vocab from a file.
  
          :param (file) f: a file object, e.g. as returned by calling `open`
          :return: a vocab object. The 0th line of the file is assigned to index 0, and so on...
          """
          word2index = {}
          counts = Counter()
          for i, line in enumerate(f):
              word, count_str = line.split('\t')
              word = word.decode('utf-8')
              word2index[word] = i
              counts[word] = float(count_str)
              if i == 0:
                  unk = word
          return cls.from_dict(word2index, unk, counts)